Individual study: Effects of prairie and barrens management on butterfly composition in the midwestern USA
Swengel A.B. & Swengel S.R. (2001) Effects of prairie and barrens management on butterfly faunal composition. Biodiversity and Conservation, 10, 1757-1785
Since European contact in North America, about 99% of tallgrass prairie has been destroyed primarily by conversion to agriculture. Pine-oak barrens have also declined to a lesser degree. If unmanaged, extant fragments of prairie and barrens often alter by overgrowth of woody species, invasion of exotic plants, and accumulation of plant litter. Thus, it is widely believed that instigating periodic 'disturbances' or appropriate ecological processes, disrupted now due to habitat destruction and fragmentation, are needed for such habitats to persist. It is often assumed that management for these ecosystems depends upon reinstating or mimicking prehistoric processes, with fire inferred to be the dominant process. Alternative theories assert that these habitats were maintained by climate, herbivory, or a combination of these factors with fire, soil, and/or topography. The effects on insects of ecosystem management with fire have become a subject both of research and controversy. Specialist butterflies of native prairie and barrens are often assumed to be fire adapted, and observations of particular butterfly populations or sites have been viewed as corroboration of that expectation. Based on other observations or interpretations, others have questioned the assumption that frequent extensive burning benefits (or does not irretrievably harm) butterflies requiring native herbaceous habitat.
Study sites and surveys: During 1990-97, 122,138 adult butterflies were recorded in transect surveys at 125 pine- oak barrens in northern Wisconsin, USA and 106 tallgrass prairies in six midwestern states grouped into three prairie sub-regions. Most prairie sites were conserved lands in private or government ownership. The barrens included conserved lands, forest reserves, military reservation, and rights-of-way (verges) for highways and powerlines. Before analysis, the butterflies were classified into three ecological subgroups: specialist of native herbaceous vegetation; grassland (widely occurring in native and degraded herbaceous vegetation); and generalist.
Analyses: Analytical questions included the following. (1) What other factors besides management have strong effects on variability of butterfly species richness and density? (2) How much do butterfly richness and density vary in response to different management types and intervals among habitat types and sub-regions? (3) Do butterfly richness and density conflict in patterns of management response? (4) How much do the responses of the total butterfly fauna correspond to those of the most habitat specialized or most abundant ecological subgroup? (5) Do butterfly management responses conflict between more and less ecologically specialized groups? (6) Are management responses of butterflies (especially specialists) consistent with or contrary to expectations based on current theories of fire dependence in their habitats? The goal was to investigate whether seeming conflicts among studies could be reconciled to some extent by accounting for differences in methodology and design. This dataset was analyzed with multiple linear regressions both by ecological subgroups and as total butterflies, and by relative density and species richness, to investigate how these different ways of ordinating the same dataset might affect the results.
Other factors besides management: Density and richness of total butterflies and the subgroups related significantly to many non management factors. Density had slightly more significant results than richness, except vice versa for weather factors. Geographical, timing, and weather variables were frequently significant. The proportion of significance for habitat variables was, in descending order: topographical diversity, vegetative type, floristic quality (always positive), and prairie size.
Management types and intervals: In comparisons of more vs. less recent burning, all significant results for most recent burning were negative. No significant negative relationships were attributed to the longest period since burning. In comparisons of burning vs. idling, all significant results in prairie favored idling, but in barrens favored burning. In comparisons of burning vs. mechanical cutting, all significant results in prairie favored cutting, but no significant differences occurred in barrens. In regressions including all management types, rotational burning (alone or combined with cutting) was significantly positive most often for generalists and never for specialists. Increasing years since last management was always negative in barrens and the southern prairie sub-region but always positive in the two northern prairie sub-regions. Significant management patterns occurred more often in prairie than barrens, which were less fragmented. Specialists were favored by haying, grazing in one northern prairie sub-region (but disfavored in the other), single wildfire (testable in barrens only), and increasing years since last treatment in one northern prairie sub-region (but disfavored in barrens).
Butterfly richness and density: Within subregion and subgroup, significant management results for density and richness never conflicted, but density had more significant results than richness.
Total butterfly fauna and ecological subgroups: In no instances were the signs opposite when total butterflies and/or any subgroup(s) significantly related to the same management factor in the same type of regression. But what was significant for one sample was often not for another. Thus, management favorable for specialists and total butterflies did not conflict, but the subgroups had varying degrees of sensitivity, rather than opposite responses.
Theory vs. observation: The results for specialist (and total) butterflies did not endorse burning over other plausible conservation managements, especially in prairie. But they also did not consistently endorse the same alternative management among sub-regions. Since only one alternative management in one sub-region was specifically designed for conservation rather than farming, these alternative managements (e.g., grazing, haying/mowing) bear further research for conservation applications. This study and the available context of other studies indicate a disparity between observation and theoretical expectation regarding insect responses to fire in open habitats in the midwestern USA.
Conclusions: Of the factors significantly affecting butterfly density and abundance, management appears most readily manipulated by conservationists. Since specialist butterflies seemed less sensitive to management in larger intact landscapes, results there should not be applied directly to more fragmented and degraded landscapes. Comparisons of burning to other feasible management activities compatible with maintaining native flora appear more relevant than comparisons to less recent burning or idling, since the existence of fire programmes implies the budget and personnel to conduct active management. To the extent that the science remains difficult or contested in interpretation, caution should be used in preserve management, both by doing no more than appears necessary to achieve specific goals of habitat maintenance and by avoiding over reliance on one management type over others.
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